The most immediately striking thing about Once I Was an Eagle is how relentless it feels. That is not an adjective readily associated with Laura Marling, although perhaps it should be. On the one hand, profile writers have a tendency to depict the singer-songwriter as rather a delicate flower: young and aristocratic (her father is the Fifth Baronet Marling of Stanley Park and Sedbury Park), perpetually heartbroken and in charge of an acoustic guitar, as well as being pale and reserved – "with her grey-blue eyes and ghostly complexion, she has the intense, windswept look of a Brontë heroine", as one interviewer put it. She is an artist who suffers her way through playing live, which she once compared to having toothache, and worries about writing lyrics that are too personal "because my little heart can't take it". On the other, for one apparently so fragile and mild, there's certainly something relentless about her work rate: at barely 23, she's on to her fourth album.

Either way, the adjective fits here. The album's opening four songs – Take the Night Off, I Was an Eagle, You Know and Breathe – pass by without a pause. Apparently recorded off-the-cuff, or at least in such a way as to deliberately and convincingly construct a sense of spontaneity, they bleed into one another, linked by a frenetic guitar motif that seems to be inspired in equal parts by Indian raga and the riff from Pink Floyd's Fearless. If you're not watching the titles on iTunes change, it all sounds like one long episodic song. In fact, taken together, the thing they most recall, in structure and tone, is one of the scabrous acoustic epics with which Roy Harper made his name in the early 70s. For a delicate flower, Marling sounds remarkably splenetic: her voice seems to have been roughened by nicotine, which fits the toughness of the lyrics. She rails away at "hippies" – apparently they "stomp empty-footed on all that's good" – and a variety of feeble ex-lovers: "Every little boy is so naive," she scoffs. One, a "freewheeling troubadour" whom some have suggested might be her former boyfriend Marcus Mumford, was, by her account, the drippiest of the lot.

When she does finally pause for breath, after the best part of 15 minutes, she comes back even more waspish. Over clattering percussion provided by producer Ethan Johns, Master Hunter not only appropriates Bob Dylan's most famous brush-off ("If you want a woman that will follow your name," she sings, "it ain't me, babe") but also his patented withering sneer, with sudden, bug-eyed emphasis on unlikely syllables: "Took me in it to the edge of in-SAAANE/ I nearly put a BULL-et in my brain." It's all unexpectedly bracing and impressively self-assured. For all the detectable influences, it sounds like an artist who's spent the past few years trying on different styles for size – she started, aged 17, modelling post-Lily Allen mockney, before moving on to Blue-era Joni Mitchell – and has found one to fit her.

Once I Was an Eagle doesn't quite maintain its opening's level of intensity for the remainder of its 63 minutes. After an orchestral interlude, the sound becomes more fleshed-out, the atmosphere more nocturnal and becalmed, and when the riff from the opening tracks returns on the closer, Saved These Words, Marling appears to be strumming her guitar rather than thrashing wildly at it as a prelude to smashing it over the guy from Mumford & Sons' head. But the quality of the songs remains almost unerringly high: the guitar on When Were You Happy? grumbles instead of raging, but the tune is lovely; Undine is a convincing stab at parched alt-country; Once deals in a pretty exquisite brand of melancholy.

Occasionally, you're reminded that, for all the snarling confidence on display, Marling is still an artist in the process of finding her own voice. Shortly after finishing the album, she moved to Los Angeles, perhaps to be reunited with her accent, which, on the evidence presented here, seems to have emigrated to America some time before she did. At one point, this gets so out of hand that she pronounces the word "verse" as "voice", which would be fair enough had she grown up in Flatbush, Brooklyn, but feels a bit affected given that she grew up in a Hampshire village called Eversley. And there are a couple of moments where she still feels like the sum total of a very tasteful record collection, where she struggles to make herself heard over the echoes of Joni Mitchell and Dylan's thin wild mercury sound. More often, though, she cuts through her influences, and rings out loud and clear; when she does, it's a very diverting sound indeed.